The present invention relates to a circuit configuration for producing exponential predistortion for a variable amplifier.
Amplifiers, for example power amplifiers or low-noise amplifiers, such as those that are normally used for radio-frequency purposes normally have a variable gain. The characteristic with which the gain can be adjusted may in this case have to be linear, or may have to follow what is referred to as a dB-linear characteristic. A dB-linear characteristic is in this case distinguished by the fact that the output power Pout in dBmW is proportional to the control voltage Ucontrol on the input side, in accordance with the following formula:
Pout[dBm]=10xc2x7log Pout[W]xe2x88x9dUcontrol 
In most widely available amplifiers, the output power is, however, directly proportional to the control voltage, rather than being logarithmically proportional. The control voltage must therefore be predistorted in an appropriate manner in a stage upstream of the amplifier, in order to obtain a dB-linear characteristic.
A circuit by which a described predistortion can be achieved is specified in the document by Professor Robert Meyer, UC Berkeley EECS 242 (NTU Course: IC775CA), titled xe2x80x9cAdvanced Integrated Circuits For Communicationxe2x80x9d, Course Notes (Lectures 1-43, Vol. I of II, University of California at Berkeley College of Engineering, 1999, Lecture 4, p. 9). In this case, the control voltage of an amplifier is first in an exponential form, in order to obtain a dB-linear characteristic overall. The predistortion is provided by using a translinear circuit which forms a temperature-stable, exponential relationship between the control variable (current/voltage) and an output variable.
The basic circuit as specified in the document cited above for producing exponential predistortion is subject, however, to the disadvantage that the ideally calculated, expontential transfer function changes as a result of process tolerances to which the components that are required to produce it are subject, and due to temperature changes during operation of the circuit. Furthermore, the specified circuit requires a relatively large number of components as well as a large transistor area associated with this. The large number of transistors required also has the disadvantage that the overall circuit can achieve only mediocre noise characteristics.
It is accordingly an object of the invention to provide a circuit configuration for producing exponential predistortion for a variable amplifier that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which predistortion is improved in terms of manufacturing and temperature-dependent tolerances.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for producing exponential predistortion for a variable amplifier. The circuit configuration contains a first controlled current path having a first diode, and a second controlled current path having a second diode and connected in parallel with the first controlled current path. Controlled currents of the controlled current paths include a common-mode current component for setting an operating point of the first and second diodes and a differential current component for providing a small-signal drive. A differential amplifier contains a first transistor having a control input connected to the first diode, and a second transistor having a control input connected to the second diode. A ratio of effective transistor areas of the second transistor and the first transistor is equal to a ratio of effective diode areas of the second diode and the first diode.
The described circuit has two diodes of different size, which are followed by a differential amplifier with two transistors of different size. In this case, the larger of the two diodes is connected to the larger transistor, while the smaller diode is connected to the smaller transistor. The diodes are driven by currents that contain a common-mode current for setting the operating point of the diode as well as a differential current that carries the actual useful signal. Output currents are produced on the output side of the differential amplifier in order to drive an amplifier with the asymmetric predistortion that is produced by the described circuit. The asymmetric predistortion and the asymmetric differential amplifier allow the present circuit to produce an approximate dB-linear transfer function. The area ratio of the first and second diodes and of the first and second transistors can in this case be matched to the respective requirements, as described in more detail in the following text. Since the described circuit can be produced with a small number of components, it can be implemented with good noise characteristics. Furthermore, the described circuit is insensitive to process technology and temperature fluctuations.
Since the diodes have different areas, this results in the diode voltages being asymmetric. The diode voltages are used for controlling a differential amplifier, at whose output the desired, predistorted input signal is produced as a differential current for an amplifier.
In order to achieve good paring, the diodes may be transistors that are connected as diodes.
In one preferred embodiment of the present invention, the current in the first and second controlled current paths is controlled by a second voltage controlled differential amplifier with a first output which is connected to the first diode, and with a second output which is connected to the second diode. The second differential amplifier is used to convert a control signal, which is in the form of a differential voltage signal and is used for controlling the output power of an amplifier, to the control currents required for controlling the diode current paths. The second differential amplifier accordingly operates as a voltage/current converter.
In a further preferred embodiment of the present invention, the second differential amplifier has two bipolar transistors, whose emitter connections are connected to one another in order to produce negative feedback via a resistor. The negative feedback for the second differential amplifier may be particularly advantageous when its deflection in the small signal range exceeds a range in which sufficiently good linearization is available even without negative feedback. In any case, it should be remembered that the controlled currents in the first and second current paths are proportional to the differential input voltages.
In a further preferred embodiment of the present invention, the ratio of the effective transistor areas and the ratio of the effective diode areas to one another are in each case set by connecting two or more identical components in parallel. By way of example, effective area ratios of the transistors and of the diodes of 2:1, 3:1, 3:2 etc. with respect to one another can thus be produced. Since identical components are in case used in both current branches, particularly good pairing can be achieved.
In a further preferred embodiment of the present invention, the first diode contains two diodes, which are connected in parallel and are each of the same type as the second diode, and the first transistor contains two transistors which are connected in parallel and are each of the same type as the second transistor, so that this results in an area ratio of 2:1 in each case. As described in detail in the following text, which describes exemplary embodiments on the basis of derivations and parametric studies, an area ratio of 2:1 results in a dB-linear transfer function for the described circuit for producing exponential predistortion for a linear preamplifier.
In a further preferred embodiment of the invention, an area ratio of 3:1 is obtained by connecting three components, or three transistors, in parallel. Overcompensation such as this may be desirable depending on the application of the circuit for producing predistortion.
In a further preferred embodiment of the present invention, the first transistor in the first differential amplifier is connected on the control side directly to the first diode and the second transistor in the first differential amplifier is connected on the control side directly to the second diode. In this case, the potential at the diode connections, which are connected to the control inputs of the transistors, controls the differential amplifier as a function of the currents flowing through the diodes.
In a further preferred embodiment of the present invention, a load diode is in each case provided in a third and a fourth current path, which are controlled by the first differential amplifier and which respectively contain a controlled part of its first or second transistor. The predistorted current which can be derived from the first differential amplifier is applied to the load diodes in order to provide compensation to the characteristic of a differential amplifier, for example a current balance, which can be connected for gain variation.
In a further preferred embodiment of the present invention, on the output side, a first differential amplifier is connected to a current balance, whose control inputs are connected to the load diodes, and on the output side, an electrical load, for example an antenna or a matching network, can be connected to the current balance. The current balance provides variable gain.
A radio-frequency signal, for example, can be supplied to one input of the current balance and is amplified in accordance with the desired, applied nominal gain.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for producing exponential predistortion for a variable amplifier, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.